In recent years, as electronic devices such as mobile phones, video cameras, and laptop computers have rapidly become smaller and smaller in size and lighter and lighter in weight, flexible printed circuit boards (hereinafter referred to as “FPCs”) have become indispensable. Meanwhile, since FPCs are highly bendable wiring board that vary in thickness from several tens of micrometers to several hundreds of micrometers, they need to have their component-mounting areas reinforced to have the strength required for components such as connectors to be mounted thereon. Therefore, normally, a partial improvement in strength is achieved by attaching a stiffener firmly to a back surface of a component-mounting area of an FPC with a thermosetting adhesive (e.g., see Patent Literatures 1 to 3).